This invention relates to a method and apparatus for fabricating a semiconductor device. More specifically, it pertains to improvements in dry etching technology.
In the fabrication of semiconductor devices, dry etching techniques are used to produce a desired pattern on the surface of a workpiece. A typical patterning technique is made up of a photolithography process and a dry etching process. A photolithography process is a process step of creating a resist pattern having remaining portions and opening portions on a workpiece surface. A dry etching process is a process step of etching away portions of the workpiece corresponding to the opening portions of the resist pattern.
Here, a step of patterning a MOSFET gate electrode is described. As shown in FIG. 18(a), a silicon dioxide layer 7 that becomes a gate oxide layer is deposited on a silicon substrate 6, a polysilicon layer 4 is deposited on the silicon dioxide layer 7, and a resist is applied onto the polysilicon layer 4 to form a resist film 5. Thereafter, a pattern having remaining portions and opening portions is formed in the resist film 5 using a lithography technique. By making use of this resist film 5 as an etch mask, the polysilicon layer 4 is dry etched. 1 is an active species. 2 is an ion. 3 is an etch product produced during the dry etching process.
The polysilicon layer 4 is selectively etched such that recesses 9 are defined according to the opening portions of the resist film 5 and projections 8 are defined according to the remaining portions of the resist film 5, in other words a recess 9 is formed underneath an opening portion and a projection 8 is formed underneath a remaining portion. Referring to FIG. 18(a), there are illustrated two types of projections, i.e., a projection 8a and a projection 8b. The projection 8a is called an isolated projection because it is isolated from its neighboring projection with a great distance. On the other hand, the projection 8b is called a closely spaced projection because, unlike the isolated projection 8a, it is located close to another projection. Additionally, there is an isolated recess 9a on the side of the isolated projection 8a. On the other hand, there is a closely spaced recess 9b on the side of the closely spaced projection 8b. When the polysilicon layer 4 is etched according to the etch mask, the projections 8a, 8b become gate electrodes. Then, the patterning of gate electrodes is completed. In a dry etching technique that uses a reactive gas as an etchant, an etch product, which is produced by reaction between a workpiece and an etch gas, by decomposition of an etch gas, or by the like, sticks to the sidewalls of the projection 8. Such adhesion prevents etching in the lateral direction, and etching proceeds mainly downward, which is called the anisotropic etching. This anisotropic etching enables the projection 8 to have nearly vertical sidewalls, so that the projection 8 and the remaining portion of the resist film 5 come to have almost the same lateral dimension. Therefore, dry etching has been considered one of the most important techniques for semiconductor fine processing.
In recent years, technology for the miniaturization of semiconductor devices has advanced, which however produces the problem that there occurs a CD difference (CD gain and CD loss) between a master pattern and a workpiece pattern in a patterning process. CD stands for critical dimension. In the photolithography process, a resist film acts as a workpiece and therefore CD difference in the photolithography is a dimensional difference between a reticle pattern and a resist pattern. On the other hand, CD difference in the dry etching process is a dimensional difference between a resist pattern and a post-dry etching workpiece pattern, in other words, the dry-etching CD difference is a difference in the lateral dimension between a remaining portion of the resist film and a projection bottom.
These CD differences result from: (i) post-bake expansion and contraction of resist materials for patterning, and focus deviation when a resist is exposed during the photolithography process, and (ii) increase in the bottom dimension of projections caused by excessive adhesion of an etch product to projection sidewalls, and decrease in the bottom dimension of projections caused by side etching due to short adhesion of an etch product to projection sidewalls during the dry etching process.
CD differences produced during the dry etching process (dry-etching CD differences) are first described. For example, when polysilicon is etched using a conventional RIE (reactive ion etching) technique, CD differences are likely to occur, especially when HBr gas is used. The isolated projection 8a suffers from a severer CD difference than the closely spaced projection 8b. For example, suppose that a polysilicon layer corresponding to a gate length of about 0.5 .mu.m is patterned. For the case of the isolated projection 8a, the amount of dimensional increase of W2 (the bottom width of the projection 8a) with respect to W1 (the width of the remaining portion of the resist film 5) is about 0.05 .mu.m (see FIG. 18(b)). That is, W2 is greater than W1 by about 0.05 .mu.m. On the other hand, for the case of the closely spaced projection 8b, the amount of dimensional increase of W2 (the bottom width of the projection 8b) with respect to W1 is close to zero (0.00 .mu.m). Generally, a single semiconductor device contains therein an array of memory cells where transistors are densely placed and peripheral circuits where transistors are sparsely placed. Depending upon the location of transistors, variations in the CD difference from a mask pattern occur. As a result, the precision of the gate length of MOSFETs in a peripheral circuit becomes poor. This produces problems with the device electrical characteristics and the yield.
In order to prevent the occurrence of severe CD differences, a technique without using HBr gas has been proposed. For example, a high-vacuum etcher (e.g., an ECR that makes use of electron cyclotron resonance and a plasma source that makes use of helicon waves) has been proposed for the reduction of CD difference.
In photolithography technology, for the reduction of CD difference, various approaches have been tried. For example, the wavelength of light beams used for exposure is shortened and the mechanical accuracy of optical systems is improved. Presently, such mechanical accuracy has been improved to such an extent that processing accuracy corresponding to the wavelength of exposure light is obtained. For example, with respect to 0.35-.mu.m design rule devices, a current standard processing accuracy is about .+-.0.04 .mu.m when i-lines are used.
It has been known that CD differences in the photolithography process vary depending on the type of pattern. Generally, isolated projections suffer from a severer CD difference than closely spaced projections. Conventionally, a reticle pattern is pre-dimensioned in accordance with an expected CD difference for CD difference cancellation to a certain degree.
Many of the semiconductor manufacturers in the world are now trying to control each process steps of the fabrication of semiconductor devices by means of computers. In other words, the semiconductor industry aims at not only improving the basic characteristics of workpieces at each fabrication process step but also achieving FMS (flexible manufacturing system) by controlling individual wafers. Therefore, the reduction of variations in the device characteristic by controlling variations in the CD difference between each wafer has been considered important. Some semiconductor manufacturers are trying to individually control fabrication process steps, to control information about each wafer, and to control variations in the product characteristic, for accomplishing the uniformity of performance of products.
In the above-described dry etching and photolithography processes, the following disadvantages are produced.
Even in a dry etching process performed by a high-vacuum etcher, an isolated projection suffers from a severe CD difference, and the isolated-projection CD difference is different from the closely spaced-projection CD difference. Conventional techniques find it hard to cancel CD differences. Variations in the CD difference are considered one of the main factors that decrease the yield when technology for the miniaturization of semiconductor devices further advances in the future. As described above, it is extremely hard for a conventional RIE-type etcher or a high-vacuum etcher to suppress variations in the CD difference.
Additionally, it is expected that control of the cross-sectional profile of projections (i.e., gate electrodes formed by etching) becomes important in the future. However, it is extremely difficult to accomplish such control.
Meanwhile, there are limits to which the wavelength of exposure light is shortened and there are also limits to which mechanical accuracy (e.g., the accuracy of lenses and the accuracy of alignment of components) is improved. Therefore, it seems impossible to expect remarkable improvements in the processing accuracy. Additionally, even if the CD difference is reduced by reducing the dimension of reticle patterns, there still occur variations in the CD difference in a photolithography process. Although it has been recognized that such variations occur, it is hard to thoroughly cancel it due to mechanical limitations. Further, the miniaturization of devices presents new problems. For example, variations in the CD difference that have been conventionally considered negligible now become critical.
If information control at wafer level is made, and if final CD differences are made to become as constant as possible between wafers by controlling dry-etching CD differences on the basis of photolithography CD difference information for each wafer, this enables the production of devices having a uniform characteristic. It is also possible to allow an isolated projection and a closely spaced projection to have almost the same finish dimension by means of reduction of the dimension of photolithography masks.
To sum up, if the sum of a CD difference produced in a photolithography process and a CD difference produced in a dry etching process is made to become fixed, this reduces the degree of variations in the CD difference between wafers.